Technical Abstract:
Progesterone treatment on d 2 and 3 of pregnancy accelerated conceptus development and uterine protein secretion and decreased uterine capacity. By contrast, treatment with mifepristone (RU486), a progesterone antagonist, on d 2 of pregnancy decreased uterine protein secretion and conceptus development. The objective of the following experiment was to determine the effect of RU486 on uterine capacity. Gilts were unilaterally ovariohysterectomized (UHO) at 160 d of age, observed for estrus starting at 200 d of age, and mated after at least one estrous cycle of normal length (17 to 23 d). Gilts then received either corn oil (CO, n = 47) or RU486 (400 mg in CO, n = 44) intramuscularly on d 2 of pregnancy. Gilts were slaughtered on d 105 and blood was collected from each fetus to measure fetal hematocrit. Each fetus, its associated placenta and each fetal heart, liver, and brain was weighed. The number of gilts remaining pregnant, mean fetal hematocrit and mean fetal heart and brain weight did not differ between treatments. Uterine capacity (litter size in UHO gilts) was significantly less (4.7 ± 0.4 and 7.3 ± 0.3, respectively; P < 0.01) in RU486-treated gilts than in CO gilts. Fetal weights (907 ± 18 and 859 ± 17, P = 0.05) and fetal liver weights (23.9 ± 0.8 and 21.5 ± 0.8, P < 0.05) were greater in fetuses of RU486-treated gilts compared to CO gilts. The number of fetuses weighing >900 g (2.5 ± 0.3 and 2.7 ± 0.3) and the number of placentas weighing >225 g (2.0 ± 0.3 and 2.2 ± 0.3, respectively) did not differ between treatments. In contrast, the number of fetuses weighing <900 g (2.2 ± 0.4 and 4.6 ± 0.4) and the number of placentas weighing <225 g (2.8 ± 0.4 and 5.0 ± 0.4, respectively) were less (P < 0.01) in RU486-treated gilts than in CO gilts. Thus, RU486 decreased uterine capacity, primarily by reducing the number of smaller conceptuses at d 105 of gestation. These results, combined with previous results, suggest that optimal uterine capacity is associated with an optimal progesterone concentration on d 2 and 3 of pregnancy.